An electrical machine converts energy between mechanical energy of the rotating rotor and electrical energy. A motor converts electrical energy into rotating mechanical torque, and a generator converts a rotating mechanical torque into electrical energy.
Different types of alternating current (AC) electrical machines are available. They can be grouped into two different types, a synchronous type machine and an asynchronous type machine.
A synchronous machine is an alternating current (AC) rotating machine whose speed under steady state condition is proportional to the frequency of the current in its armature. The magnetic field created by the armature currents rotates at the same speed as that created by the field current on the rotor, which is rotating at the synchronous speed, and a steady torque results. Synchronous machines are commonly used as generators especially for large power systems, such as turbine generators and hydroelectric generators in the grid power supply. Because the rotor speed is proportional to the frequency of excitation, synchronous motors can be used in situations where constant speed drive is required.
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes. Their speed can be controlled with a variable frequency drive also known as a power converter or a frequency converter.
When the magnetic field that excites current flow in stator windings of an electrical machine is provided by means of a permanent magnet (as opposed to a coil or winding in the rotor), the machine is known as a permanent magnet synchronous machine. Thus, a “permanent magnet generator” as the term is used herein, is a generator that has a plurality of stator windings and one or more permanent magnets whose magnetic field excites current flow in the stator windings during operation.
An electrical machine can end up in a failure mode with different fault scenarios. Typical examples of fault scenarios include:                open-circuit of a stator phase (e.g., a connecting cable is broken or an open circuit breaker or open contactor)        short-circuit phase to ground (e.g., insulation failure because of mechanical damage, or damage due to e.g. partial discharge)        short-circuit of one or more phase windings (e.g., insulation failure because of thermal stress within the stator or rotor, or damage due to e.g. partial discharge)        
There are several reasons to short circuits faults in electrical machines; common for all of them is the importance of disconnecting the electrical machine from the electrical supply whenever a fault is detected. A generator without a magnetic field can't produce any power even if there is a mechanical torque on the shaft. Whereas a permanent magnet generator always has full magnetic field all the time, because of the permanent magnets, and thus it is capable of producing electrical power whenever there is a mechanical torque on the shaft. The possibility to demagnetise the machine via a control circuit of the excitation current to the rotor circuit does not exist on permanent magnet machines.
In renewable energy power plants it is more and more common to use permanent magnet generators, this can be in wind turbine generators, wave power plants or others where an energy conversion from mechanical to electrical energy is needed.
If a short circuit fault in a permanent magnet generator is not detected it can cause fire in the generator and its surroundings, thus it is important to be able to detect a short circuit fault in a permanent magnet generator. The present invention presents a solution to protect permanent magnet generators.